1. Field of the Invention
The present invention relates in general to a system for compensating for thermal drift of a logic circuit output signal.
2. Description of Related Art
A typical digital logic circuit produces an output signal in delayed response to an input data or clock signal. It is often desirable that the logic circuit provide a fixed delay between the input signal and the output signal. However the delay between input and output signals will often be somewhat dependent on the temperature of the integrated circuit implementing the logic circuit because the switching speed of gates forming the logic circuit is temperature dependent, particularly for CMOS integrated circuits. This temperature dependency of the timing of the output signal with respect to the clock signal is known as "thermal drift".
Some prior art thermal drift compensation systems attempt to control thermal drift by maintaining an integrated circuit at a constant temperature. Such systems include a heater for warming the integrated circuit and some means for sensing the temperature of the integrated circuit and for turning on the heater when the circuit temperature falls below a desired set point. Such systems are expensive and difficult to implement.
Other prior art systems control thermal drift by adjusting the voltage of the power supplied to transistors forming the logic circuit. The higher the supply voltage, the faster the transistors switch. Switching speed can be sensed, for example, by sensing the frequency of oscillation of a ring oscillator formed on the same integrated circuit with the logic circuit and constructed of similar transistors. When the oscillator frequency falls below or rises above a desired set point, the power supply voltage to the transistors of the oscillator is increased or decreased accordingly so as to maintain the oscillator frequency at the set point. Since the power signal also supplies the transistors forming the logic circuit, by stabilizing the oscillator frequency we ensure that the switching speed of the logic circuit remains constant despite variations in temperature of the integrated circuit. This system controls thermal drift well but the circuitry required to provide an adjustable analog power supply signal to the integrated circuit is difficult and expensive to implement.
What is needed is a simple and effective system for compensating for thermal drift of a logic circuit which can be implemented with digital circuitry that does not have to control analog power signals and which does not require internal or external heaters.